1. Field of the Invention
This invention relates to a process for the production of an optical fiber bundle for heat resistance and vacuum resistance which can be used even at a very high temperature or high vacuum.
2. Description of the Prior Art
Using optical fiber filaments each having a core part and clad part, a number of these filaments are uniformly bundled for transmission of lights to prepare image guides or light guides in such a manner that the optical axes are properly arranged. These devices have wide appliances and are used as an endoscope for medical treatment, illumination means of a narrow space, light monitor, optical sensor, etc., which are important individual optical parts of optical devices, apparatus and other precision devices and apparatus. As such optical fiber bundles, there are used those respectively composed of a fixed part at the end part and a flexible part at the middle part.
The fixed part of the end part is formed by mainly fixing with an organic or inorganic adhesives, for example, by methods using organic adhesives as disclosed in Japanese Patent Laid-Open Publication Nos. 111908/1980 and 262105/1985 and using inorganic adhesives, in particularly, glasses as disclosed in Japanese Patent Laid-Open Publication Nos. 65907/1980 and 114804/1989.
flowever, the adhesives for fixing the end part of the optical fiber bundle, of the prior art, are not suitable for use at a higher temperature, since the heat resistance temperature of organic adhesives, for example, epoxy type resin adhesives is about 150.degree. C. and that of polyimide type resin adhesives is about 300.degree. C., as disclosed in the above described publications. Furthermore, even in use for vacuum resistance, the use of organic adhesives results in the problem that voids or pores are formed by the adhesive reaction process of the resin to lower the gas tightness and during evacuation, a gas is generated to lengthen the time for the evacuation.
When using inorganic adhesives, in particular, glasses, the gas tightness at bonded parts is good and no gas is generated, so that the adhesives of this type can well be applied to a use needing vacuum resistance. However, since the glass used as an adhesives is a low melting point glass, there arises the problem that the heat resistance temperature is also only about 300.degree. C. similar to the organic adhesives and this adhesives cannot be applied to a use at a higher temperature.
Regarding this problem of heat resistance, there has been proposed a method comprising using a quartz-type optical fiber as an optical fiber filament and an organic binder, in which an inorganic component is dissolved, as an adhesives and thereby raising the heat resistance temperature employed to a very high temperature, e.g. 800.degree. C.
However, this method has also the disadvantage that since in a bonding process, the organic component is removed by a heat treatment after the bonding, voids are also formed similarly to the case of using resin adhesives to lower the gas tightness and to decrease the bonding strength and thus bonded parts are brittle. In addition, owing to the use of the quartz-type optical fiber as an optical fiber filament, the production cost is very high.
Furthermore, as disclosed in the above described laid-open publication, it has been known to immerse the end of an optical fiber bundle in a low melting point glass, thus effecting fusion bonding. In this method, however, a large quantity of glass adheres to the end of the optical fiber bundle, expands and immediately solidifies, so that it is considerably difficult to draw the optical fiber bundle in, for example, a metallic tube for fixing the end part. The present invention has been made for the purpose of solving the problems of the prior art.